The hollow structure is a remarkable feature of carbon nanotubes (CNTs) as described in Iijima S., Nature (London) 354, pp. 56-8 (1991) since it provides a significant chance to use the carbon nanotubes for the channeling of high energy charged particles and associated channeling radiation. In particular, theoretical studies of particle channeling through CNTs have recently demonstrated that a particle beam channeled in a nanotube could be efficiently steered in the way similar to crystal channeling, which could be used as small nanobeams for biological studies and medical therapy. See Krasheninnikov A. V. & Nordlund K. Phys. Rev. B 71, 245408 (2005); Zhou D. P., Song Y. H., Wang Y. N., & Miskovic Z. L. Phys. Rev. A 73, 033202 (2006); Bellucci S., Biryukov V. M., Chesnokov Y. A., Guidi V., & Scandale W. Nucl. Instrum. Methods Phys Res. B 202 pp. 236-241 (2003); and Moura C. S. & Amaral L. J. Phys. Chem. B 109, pp. 13515-13518 (2005).
However, the nano size of the CNTs makes it difficult to precisely control the position and the orientation of the CMT. As a result, the experimental feasibility of channeling through CNTs has not been demonstrated. Only one research group from Germany is known to have worked on observing very short (15-60 nm) multiwall CNTs in the normal direction under conventional TEM as described in Kruger A., Ozawa M., & Banhart F. Appl. Phys. Lett. 83, pp. 5056-5058 (2003).
Since multi-wall carbon nanotubes are flexible and usually bent, it is necessary to produce short and straight nanotubes that can be aligned perfectly with their axes along the electron beam. Stolterfoht et al. in Germany achieved this goal by the following two ways: (i) Multiwall CNTs from the cathode deposit in an arc-discharge were embedded in epoxy and cut normal to their axes in an ultramicrotome with a diamond knife into slices of approximately 60 nm in thickness; and (ii) very short multiwall CNTs with lengths of 15-20 nm were produced with an arc discharge between two graphite electrodes operated under water. They successfully examined the short CNTs from the axial direction under conventional TEM and observed the evidence for a Fresnel diffraction effect. From their CNT sample preparation process it is clear that their CNT channel is randomly oriented and the CNT channel has to be extremely short (less than 100 nm) to be appeared straight for the transmission electron microscopy (TEM) observation. This made fabrication of individual CNT collimators impossible.
Recently a chemical vapor deposition synthesis of a monolithic multiwall carbon nanotube with a graphitic shield was described in Kleckley S, Chai G. Y., Zhou D., Vanfleet R., & Chow L., Fabrication of multilayered nanotube probe tips, Carbon 2003; 41: pp. 833-6. This unique structure of fiber coated CNT (F-CNT) is ideal for the CNT channeling experiments. With recently developed focused ion beam (FIB) assisted CNT device fabrication technique described by Chai G., Chow L., Zhou D., & Byahut S. R. Carbon 43, 2083-7. (2005), co-inventors of the present invention successfully fabricated two individual CNT collimators with inner diameters both about 20 nm and CNT column lengths 700 nm and 3 micron respectively.
The F-CNT structure makes the alignment of the CNT core straightforward by manipulating the micron size carbon fiber. The fiber shield also provides protection for the incident beam damage to the CNT from the focused ion beam (FIB) fabrication process. Unfocused electron beams are successfully propagated through the CNT collimators in a conventional TEM instrument as described in a paper by Chai G., Heinrich H., Chow L., & Schenkel T., submitted to Science (2007). The Fresnel diffraction phenomenon was observed and by tilting the F-CNT package with 1 degree the CNT channel is optically blocked. However, some electron transmission through the CNT core was observed, clear evidence of the electron channeling through the CNT column.
The demonstration of efficient electron transport through a single, micrometer long, and well aligned carbon nanotube has the potential to realize new classes of collimators and beam optics for energetic particles—ions as described in Persaud A., Park S. J., Liddle J. A., Rangelow I. W., Bokor J., & Schenkel T. NanoLetters 5, 1087 (2005), as well as electrons. The use of fiber coated carbon nanotubes makes the handling of single tubes robust and compatible with standard micromanipulation techniques, and testing that a tube is really open with widely available TEMs enables transmission and material transport experiments through CNTs with much higher rates of reproducibility and less frustration then previously possible.